This invention relates to edge-guiding of tape approaching and leaving a helical path on a drum, and particlarly such guiding adapted to conduct the tape into and from paths that are parallel to a reference plane with respect to which the drum is tilted.
In U.S. Pat. No. 3,189,289 there is disclosed a magnetic tape guiding structure in which the tape is guided to and from a helical path on a scanning drum to a path that is parallel to a predetermined reference plane. This solution employs a number of right-cylindrical roller guides engaging the tape tangentially along generatrices that are normal to the tape centerline, each guide being skewed with respect to the adjacent guides, but only in planes that are parallel to one another, so as to twist the tape about its centerline without causing it to wrap helically around the rotating guides. In this structure the drum is simply normal to the reference plane. The present invention utilizes a similar principle but for a different and more complex purpose, which is to provide smooth planar edge-guiding of the tape in either a fixed or rotating guide system; and the present invention utilizes only a single intermediate guide between the reference plane and either side of the helix, with the additional complication that the helical drum is tilted with respect to the reference plane.
U.S. Pat. No. 3,691,315 discloses a system for guiding magnetic tape between a helical path on a scanning drum and a reference plane, with only one intermediate guide on either side of the helix. In this structure, the helical scanning drum also embodies an additional complication in being tilted with respect to the reference plane. The intermediate guide is mounted to be axially parallel to the drum, which simplifies its mounting, but which renders it less suitable for edge-guiding purposes, because the tape wrap thereabout is helical in form, and edge-guiding flanges, if mounted thereon, would have to be non-planar in form; that is, they would have to follow a complex helically warped surface in order to avoid undue pressure on the tape edges. Planar edge guides have been tried on such guides, but it has been found that they engage the tape edges at only one or two tangential points, at which all of the edge-guiding forces become concentrated, resulting in undue wear and damage to the tape, and inaccurate guiding, caused by variable curling of the tape edges. It is true that in the environment disclosed, in which two magnetic scanning heads are used, the tape warp being only 180 degrees, it is quite satisfactory to mount the edgeguiding flanges as shown on those cylindrical guides that are nearest to the drum, and around which the tape is wrapped in a plane normal to the guide. However, for use with drums having only one rotating scanning head, around which the tape is wrapped helically for 360.degree. or slightly less, and for uses in which the results must meet extremely rigorous standards, it is preferable to mount the edge guides on those guides that are closest to the entrance and exit points of the helical tape path around the drum. The present invention, in addressing itself to this problem, combines the solutions taught by U.S. Pat. No. 3,189,289 to Maxey and U.S. Pat. No. 3,691,315 to Ellmore.
U.S. Pat. No. 3,432,084 to Van Den Bussche teaches a guiding system in which the tape is bent around the equivalent of a guide post in a helix and without the use of edge guiding flanges.
U.S. Pat. No. 3,940,791 to Kayan et al. and U.S. Pat. No. 3,414,684 to Lichowsky each shows a helical guiding system for solving a simpler problem, in which the tilt of the drum is in the same plane as the helix angle inclination of the tape.
Accordingly, it is an object of the present invention to edge guide tape between planar and tilted helical paths, with least wear and damage to the tape edges.